1. Field of the Invention
This invention relates to optical measuring devices, and more particularly to improvements in an optical measuring device, wherein parallel scanning ray beams are utilized to measure dimensions and the like of a workpiece to be measured.
2. Description of the Prior Art
Heretofore, there has been adopted an optical measuring device wherein rotary scanning ray beams (laser beams) are converted by a collimator lens into parallel scanning ray beams being passed through this collimator lens and a condensing lens, a workpiece to be measured is interposed between the collimator lens and the condensing lens, and the dimensions of the workpiece to be measured are measured from a time length of a dark portion or a bright portion generated due to the obstruction of the parallel scanning ray beams by the workpiece to be measured.
More specifically, as shown in FIGS. 1 and 2, laser beam 12 are oscillated from a laser tube 10 toward a stationary mirror 14, the laser beams 12 reflected by the stationary mirror 14 are converted into rotary scanning ray beams 17 by a polygonal rotary mirror 16, the scanning ray beams 17 are converted into parallel scanning ray beams 20 by a collimator lens 18, a workpiece 24 to be measured interposed between the collimator lens 18 and a condensing lens 22 is scanned at high speed by the parallel scanning ray beams 20, and the dimension of the workpiece 24 to be measured in the scanning direction (direction Y) is measured from the time length of the dark portion or the bright portion generated due to the obstruction of the parallel scanning ray beams by the workpiece 24 to be measured. More specifically, the bright and dark portions of the parallel scannning ray beams 20 are detected as variations in input voltage of a light receiving element 26 disposed at the focal point of the condensing lens 22. Signals from the light receiving element 26 are fed to a pre-amplifier 28, where they are amplified, and then, fed to a segment selector circuit 30. This segment selector circuit 30 is adapted to generate a voltage V from the voltage outputted from the light receiving element 26 to open a gate circuit 32 only for a time t, during which the workpiece 24 to be measured is scanned, and feeds the same to the gate circuit 32. A continuous clock pulses CP are fed to this gate circuit 32 from a clock pulse oscillator 34, whereby the gate circuit 32 generates clock pulses P for counting the time t corresponding to a dimension in the scanning direction (i.e., an outer diameter) of the workpiece 24 to be measured and feeds the same to a counter circuit 36. Upon counting the clock pulses P, the counter circuit 36 feeds a count signal to a digital indicator 38, where the dimension in the scanning direction of the workpiece 24 to be measured is digitally indicated. On the other hand, the rotary mirror 16 is rotated by a synchronous motor 44 driven in synchronism with an output from a synchronous sinusoidal wave oscillator 40 for generating a sinusoidal wave in synchronism with an output from the clock pulse oscillator 34 and a power amplifier 42 in response to clock pulses CP emitted from the clock pulse oscillator 34, whereby the measuring accuracy is maintained.
The above-described fast-scan type laser length measuring device has been widely utilized because the lengths, thicknesses and the like of moving woekpiece and workpieces heated to a high temperature can be measured at high accuracies in non-contact relationship therewith.
However, in the case of the above-described fast-scan type laser length measuring device, since a reflecting point 16A of the rotary mirror 16 is periodically varied in its distance in the scanning direction to an optical axis 18A of the collimator lens 18 as enlargedly shown in FIG. 3, such a disadvantage is presented that the measuring accuracy fluctuates.
Furthermore, in the above-described fast-scan type laser length measuring device, if a bedplate, to which are secured the stationary mirror 14, the rotary mirror 16 and the collimator lens 18, is expanded or shrunk due to a change in the ambient temperature, then a distance from the rotary mirror 16 to the collimator lens 18 is varied accordingly, whereby the reflecting point 16A of the rotary mirror 16 is varied in the scanning direction with respect to the optical axis 18A of the collimator lens 18, thus presenting the disadvantage of lowering the measuring accuracy.
Additionally, when the above-described length measuring device is assembled or adjusted, the adjustment of positional relationship between the stationary mirror 14, the rotary mirror 16 and the collimator lens 18 has been very difficult, thus affecting the measuring accuracy to a considerable extent.
To obviate the disadvantages that the bedplate is expanded or shrunk due to the temperature change, there is a proposal that the bedplate, on which are rested the stationary mirror 14, the rotary mirror 16 and the collimator lens 18, is made of an alloy or the like being low in the coefficient of thermal expansion, however, in this case, such alloy materials are high in cost, thereby presenting the disadvantage of increasing the cost to a considerable extent.